What's the best way to deal with Co-workers?

Or Co-worker, I should say.

I work in an office and I really like my job. The work is enjoyable and the people are super nice. But, one co-worker just is so lazy but so entitled that it's driving me crazy. Yesterday, most of us worked late to prepare for a major meeting that's on Friday. I don't mind it at all, but our general manager got us both together to ask why a certain assignment hadn't been completed. She was given that assignment and I knew nothing about it. But instead of owning up to her error, she looks at me anId proceeds to blame the entire thing on me. I was surprised, needless to say. I said that I wasn't aware of the assignment but I'd be more than willing to finish it. It took there days. This is the second time this has happened and it puts me behind on my own work.

We both have entry level positions and she's been with the company 2 months longer than I have. Because of this, when she tries to “assign" me work like she's my superior, I get annoyed mainly because I'm fixing her mistakes on top of my work while she's shopping for cars online or tweeting or complaining about Mr not finishing what was her task.

Basically, I'm just wondering how to best deal with this. I don't want to complain to our GM, and I'd rather us deal with it, but her attitude OS getting ridiculous.